This invention relates to methods and apparatus for the introduction of fibrous material into a substance or mixture. In particular but not exclusively, the invention relates to such methods in which cellulose fibres, other organic fibres or mineral fibres are provided in pelletised form for subsequent addition to mixture of ingredients.
Cellulose or mineral fibre are commnonly used to modify the rheological properties of liquid systems, including those based on cement and water, and those based on bitumen. The fibres have a gelling or thickening effect on these systems which may include, for example, a cement-based tile adhesive or a bituminous road surfacing compound. Cellulose and mineral fibre usage has increased partly due to the demise of asbestos fibre.
Cellulose fibre may be derived for example from recycled paper, timber, woodpulp, or various other agri-materials such as straw, flax. Other organic fibres include, for example, polyester, polythene and polypropylene. Mineral fibre is commonly made from glass, basaltic rock or steel slag.
Fibre is normally supplied in compressed bales which may be from 1 Kg to 400 Kg in weight. There are three main methods for adding fibre to the mixes. In one such method, bales may be added direct to a mixer. Thus, in the case of hot bitumen mixes, the whole bale (including polyethylene packaging) may be added, the plastic quickly dissolving into the hot liquid bitumen. Alternatively, and particularly in the use of dry powder mixes, the fibre is emptied from the packaging direct into the mixer, or perhaps via a hopper and screw arrangement, possibly after a weighing operation to obtain the exact quantity required for a batch. These methods are labour intensive and often require an operative to work very close to mixing machinery, with environmental problems of dust, fumes and noise.
In a further method, bales of fibre may be emptied into a pneumatic blowing machine and dosed gravimetrically or volumetrically into the mixer along a pipe which may be 25-150 mm in diameter for example. This method lends itself to the use of bigger bales with consequent reductions in labour intensity, and also provides a solution to the environmental problems. However, it still falls short of true bulk handling.
In another method, the fibre may be pelletised, or granulated possibly by the fibre manufacturer, in a pelletising or granulating process in which small compressed pellets or granules are formed by extrusion, pressing or other available method. Normally, an additive is used which lubricates this process and acts as a binder. This additive may be bitumen, wax, oil or other suitable materials. However it is also possible to pelletise or granulate some fibres without any additives. The pellets or granules lend themselves to true bulk handling. They can be delivered in bulk loads of up to 20 tonnes and blown into a silo in a similar method to that used for animal feed pellets for example. From the silo, pellets or granules can be screwed, conveyed, dosed or weighed as required in a fully automatic manner without labour costs. However, the use of pellets or granules is very often not preferred because it is difficult to achieve an effective dispersion of the fibre due to the work required in the mixer to break up the hard-compressed pellets or granules into their constituent fibres.
Where pellets or granules are used, as a component of powder or liquid mixes, the mixer is relied upon to achieve a fortuitous breakdown of the pellets or granules into dispersed fibre by shear, attrition and abrasion effected by the other components of the mix which may include stone aggregate for example, the mixer paddles themselves and possibly by the elevated temperature of the mixture causing the binder material if present to soften, or to diminish the binding effect. Dispersion of fibre in this way is not what the mixer was designed to achieve and, therefore, it performs the task usually inefficiently. The result can be longer mixing times with consequent reduction in plant capacity and increased wear and tear and power consumption. Even then, dispersion may be incomplete resulting in non-homogeneous mixes and inconsistent thickening effect.
We have therefore designed an improved method for the addition and substantially uniform dispersion of fibres in a mixture which overcomes or mitigates at least some of the disadvantages of the above methods. In particular it provides a method in which the fibrous material is initially rendered into compact pelletised or granulated form for efficient storage, handling and transport and then returned to an open fluffy state at the mixing station. This considerably improves storage, transport and handling of the fibre as it is in a compact form which can be transported and handled in bulk and which also can be metered automatically into a mixture using a screw conveyor and the like.
The process also includes the novel step of refibreising the pellets or granules prior to addition to the mixture. Thus the method as a whole includes the innovative feature of temporarily reducing a fibrous material to a compact pelletised or granular state for storage, transport, handling and metering and thereafter returning it to an open-textured state prior to mixing.
In one aspect, this invention provides a method of producing a substantially uniform dispersion of fibrous material in a mixture, which comprises:
providing a supply of pellets or granules of compacted fibrous material;
refibreising said pellets or granules to cause at least some of said pellets or granules to expand to a relatively open-textured fibrous material; and
introducing said refibreised fibrous material into said mixture and mixing it therewith.
In another aspect, this invention provides a method of producing a substantially uniform dispersion of fibrous material in a mixture at a mixing station which comprises:
pelletising or granulating at a pelletising or granulating station a fibrous base material by at least one of compaction and possibly binding of the fibres of the fibrous base material;
transporting said pelletised or granular base material to a remote refibreising station;
refibreising said pelletised or granular base material to recover a relatively open-textured fibrous material; and
introducing and mixing said refibreised fibrous material into said mixture.
In each aspect, said fibrous material preferably comprises cellulose or other organic or mineral fibres or a mixture thereof. Cellulose fibre may be derived from e.g. recycled paper, timber, woodpulp or various other agri-materials such as flax or straw. Organic fibres may be of polyethylene, polyester, or polypropylene. Mineral fibres may be derived from glass, basaltic rock or steel slag.
Preferably, said refibreising is performed by means of one or more of grinding hammer-milling, disc-refining, pulverising and air impaction or other means of attrition. The preferred objective is to separate the fibres by mechanical means without causing unnecessary damage to the fibres themselves. The opening and re-fibreising of the material may continue after addition to the mixture, by the mixing action applied thereto.
Said mixture may comprise a wide range of different materials, but this method has been designed with particular reference to the addition of fibrous material into a liquid-based system to improve its rheological properties, for example those based on cement and water (e.g. a cement-based tile adhesive) and those based on bitumen (e.g. road-surfacing compounds).
In another aspect, there is provided apparatus for the introduction of a generally open-textured fibrous material into a mixture, said apparatus comprising:storage
means adapted to store in use a supply of pelletised or granulated fibrous material;
a fibreising means adapted to expand said pelletised or granular material to a generally open-textured form;
means for delivering in use said pelletised or granular fibrous material from said storage means to said fibreising means;
means for conveying said expanded open-textured fibrous material from said fibreising means, and introducing it into said mixture.
In yet a further aspect, this invention provides a method for the transport and metering of a fibrous additive material into a mixture, which method comprises reducing said fibrous additive material from an open-textured form into compact pelletised or granular form, transporting said pelletised or granular fibrous additive material to a mixing station, returning said fibrous material from said compact pelletised or granular form to an open-textured form, and thereafter adding said open-textured material to said mixture.
Whilst the invention has been described above, it extends to any inventive combination of features set out above or in the following description.